Re: Starship Deckplans
You tool of choice is probably going to depend on how you want the program to work.
If you want a CAD-like design program, CC2 (not free) or Autorealm (free) are your best choices. They're both designed to do RPG mapping.
If you want a program that works like a paint program (like photoshop), try the GIMP. It's a powerful, general purpose paint program, and it's free. The interface isn't as friendly as photoshop or paintshop pro, but you can't beat the price.
All of those tools require two things: Artistic talent and a willingness to climb the learning curve. None of them are going to have you churning out beautiful maps in a few hours.
My advice for getting started is to rough out your first designs on paper first! Don't try to do your creative work at the same time you're learning your tools. Start with a concept, then use the program to put it into pixels. That way, you're only dealing with the learning curve of the software.
Good Luck